1. Field of the Invention
The present invention generally relates to wideband antennas with compact and planar geometry and, more particularly, to planar inverted cone and fourpoint antennas.
2. Background Description
The need for wideband antennas with omnidirectional coverage is increasing in military and commercial applications. Thin antennas are preferred in most situations. The classic solution is to obtain an omnidirectional pattern uses a thin wire dipole or its counterpart monopole version with a ground plane (if a half-space is to be eliminated). However, the wire dipole and monopole suffer from narrow impedance bandwidth. The bandwidth can be widened by using flat metal rather than a thin wire structure. Many flat radiator geometries have been explored over several decades. However, most such antennas suffer from pattern degradation at the high end of their impedance bandwidth.
Crossed half circle flat radiators have also been investigated and appear to provide better patterns within impedance bandwidth, but simulation results reveal that they have high cross polarization over the entire band due to the interaction between flat elements.
A flat circular disc antenna was used as a TV antenna operating at 90–770 MHz and described by S. Honda in 1992. (S. Honda, M. Ito. H. Seki and Y. Jinbo, “A disc monopole antenna with 1:8 impedance bandwidth and omnidirectional radiation pattern”, Proc. ISAP '92 (Sapporo, Japan), pp. 1145–1148, September 1992). The circular disc antenna is composed of a flat circular disc 1 mounted above and perpendicular to a ground plane 2 as shown in FIG. 1. The circular disc antenna has a very large impedance bandwidth, about 10:1. A circular disc antenna of diameter A=25 mm, made of 0.5 mm thick brass plate mounted at height h=0.7 mm over a square ground plane (30 cm×30 cm) yielded acceptable impedance (VSWR<2) over the operating band from 2.25 to 17.25 GHz for a bandwidth of 7.7:1 as shown in P. P. Hammoud and F. Colomel. “Matching the input impedance of a broadband disc monopole”, Electronic Letters, Vol. 29, pp. 406–407, February 1993. However, the radiation patterns of the circular disc antenna degrade at the high end of the band. The direction of the conical beam maxima in the E-plane pattern vary from 30° to 60° in elevation as frequency increases from 2.5 to 9.0 GHz, whereas in the H-plane the pattern remains somewhat omnidirectional with maximum variation in azimuth increasing from 4 dB to 7 dB over the band as described in N. P. Agrawall, G. Kumar, and K. P. Ray, “Wide-band Planar Monopole Antennas”, IEEE Transactions on Antennas and Propagation, Vol. 46, No. 2, pp. 294–295, February 1998.
Several modified flat monopole antennas were proposed by N. P. Agrawall, G. Kumar, and K. P. Ray in “Wide-band Planar Monopole Antennas”, IEEE Transactions on Antennas and Propagation, Vol. 46, No. 2, pp. 294–295, February 1998, to obtain better impedance bandwidth. They are elliptical, square, rectangular, and hexagonal shaped flat monopoles. An elliptical disc monopole antenna having an ellipticity ratio of 1.1 yields the best performance. However, the modified flat monopole antennas still suffer from radiation pattern degradation in E-plane.
A trapezoidal shape flat monopole antenna shown in FIG. 2 has been proposed as a variation of square flat monopole antenna by J. A. Evans and M. J. Ammann, “Planar Trapezoidal and Pentagonal monopoles with impedance bandwidth in excess of 10:1”, IEEE International Symposium Digest (Orlando), Vol. 3, pp. 1558–1559, 1999. The trapezoidal radiating element 3 is mounted above and perpendicular to the ground plane 4. The impedance bandwidth of the antenna was optimized by tapering the lower base 5 near the ground plane 4. However, the trapezoidal flat monopole antenna does not solve the problem of variations in tilt angle of the E-plane pattern peak.
A crossed half disc antenna shown in FIGS. 3A and 3B was proposed as a variation of the bow-tie antenna described by R. M. Taylor, “A broadband Omnidirectional Antenna”, IEEE Antennas and Propagation Society International Symposium Digest (Seattle), Vol. 2, pp. 1294–1297, June 1994. The crossed flat (i.e., planar) elements 6, 7 and 8, 9 improve the antenna pattern over the impedance bandwidth compared to a single half disc element. The dotted circle inside of the half disc 7 in FIG. 3B represents the size of a circular disc having similar impedance bandwidth. The crossed half disc antenna is about double the size of the circular disc antenna.
Typical specification for omnidirectional antennas from 0.5 to 18 GHz require ±2.0 dB pattern variation from omnidirectional, 0 dBi gain, and 3:1 Voltage Standing Wave Ratio (VSWR). The crossed half disc antenna of FIGS. 3A and 3B maintains the pattern and gain specifications over a much broader bandwidth, with a 2:1 VSWR from 0.5 to 18 GHz.
However, cross polarization can be high. Additionally, there are many applications in both industry and government for a wideband, low-profile, polarization diverse antenna. Communication systems, including commercial wireless communications, often require antennas that cover several frequency bands simultaneously. Another desirable feature is that of dual polarization to support polarization diversity, polarization frequency reuse, or polarization agile operation.
Wideband antenna research at VTAG (Virginia Tech Antenna Group) began in 1994 and has resulted in several inventions. Of specific interest are two patents for the Foursquare antenna: J. R. Nealy, “Foursquare Antenna Radiating Element,” U.S. Pat. No. 5,926,137, and Randall Nealy, Warren Stutzman, J. Matthew Monkevich, William Davis, “Improvements to the Foursquare Radiating Element-Trimmed Foursquare,” U.S. Pat. No. 6,057,802.
The operating band of an antenna spans a lower operating frequency fL to an upper operating frequency fU. The center frequency is denoted as fC=(fU+fL)/2. The operating band limits fL and fU are determined by acceptable electrical performance. For wideband antennas, this is usually the input VSWR referenced to a specified impedance level. For example, a popular specification is the VSWR≦2 over the band fL to fU for an input impedance of 50 Ω. Bandwidth defined as a percent of the center frequency is Bp=(fU−fL)/fC×100%. Bandwidth defined as a ratio is Br=fU/fL.
The Foursquare antenna, as described in U.S. Pat. No. 5,926,137, is shown in FIGS. 17A and 17B. It comprises four square radiating elements 11, 12, 13, and 14 on the top side of a dielectric substrate 15 which is separated from a ground plane 16 by a foam separator 17. At least two coaxial feeds 18 and 19 connect to interior corners of opposing pairs of radiating elements. This Foursquare antenna provides wideband performance and several practical advantages for commercial and military applications. Its features are a low-profile geometry, dual polarization, compact radiating element size; these features make it ideal for use as an array element. The Foursquare antenna provides dual, orthogonal polarizations naturally, but these polarization outputs can be processed to produce any polarization state.
The diagonal length, √{square root over (2)}A, of the antenna is about λL/2 and the height “h” of the element above the ground plane is about λU/4, where λL and λU represent wavelength at the lower and upper operating frequencies fL and fU.
Several Foursquare antenna models have been constructed and tested. FIGS. 18A and 18B show the computed and measured impedance and VSWR (Voltage Standing Wave Ratio) curves of the Foursquare antenna in FIGS. 17A and 17B with the dimensions listed in Table 1.
TABLE 1DescriptionSymbolSizeElement side lengthA21.3mm (0.84″)Substrate side lengthC21.8mm (0.86″)Gap widthW0.25mm (0.01″)Substrate thicknessts0.7mm (0.028″)Foam thicknesstd6.4mm (0.25″)Element height aboveh7.06mm (0.278″)ground planeFeed position distanceF′4.3mm (0.17″)
A dielectric constant 2.33 of the dielectric substrate was used in both simulation and measurement. The Foursquare antenna was simulated using the Fidelity code from Zeland software (Fidelity User's Manual, Zeland Software Inc., Release 3, 2002). Fidelity uses the Finite Difference Time Domain (FDTD) method to perform numerical computation. The measured and calculated impedance associated VSWR (into 50 Ω) are plotted in FIGS. 18A–B. The agreement between measured an calculated results indicates that accurate studies can be performed by simulation. The resistance of the antenna is about 50 Ω over the operating band and the reactance of the antenna is mostly inductive.
FIGS. 19A and 19B show the measured radiation patterns of the Foursquare antenna at 6 GHz. The E-plane pattern is the radiation pattern measured in a plane containing feed; see FIGS. 17A and 17B. The H-plane pattern is the radiation pattern in a plane orthogonal to the E-plane. The patterns at other frequencies are similar to the patterns at 6 GHz in FIGS. 19A and 19B.
U.S. Pat. No. 5,926,137 also shows a cross-diamond antenna as a modification of the basic Foursquare antenna. The construction of the cross-diamond antenna is the same as Foursquare antenna. The cross-diamond radiating elements are shown in FIG. 8 of U.S. Pat. No. 5,926,137 and comprise four diamond-shaped metal plates with included angles α1 and α2, that may the be the same or different, depending on the application. A test model with the same outer dimensions with the Foursquare antenna listed in Table 1 and with angles α1=60° and α2=59.76° was constructed and measured. The measured data demonstrated that the cross-diamond antenna may be used in the same applications as the Foursquare antenna and has a bandwidth intermediate between conventional dipole antenna and the Foursquare antenna.